Australia is in the grip of its first heatwave of the summer, with the Bureau of Meteorology warning holidaymakers to stay hydrated and avoid prolonged exposure to the sun.

"The immediate threat of severe thunderstorms has passed, but the situation will continue to be monitored and further warnings will be issued if necessary," it said in a 4.38pm update.

Meanwhile, the slow-moving high-pressure system that brought blistering heat to Perth during this week's Ashes Test is now sending the mercury into the high 30s or low 40s across much of the continent.

The Sun sets over Lake Burley Griffin as temperatures hit above 30 degrees in Canberra.

The sun sets over Lake Burley Griffin as temperatures hit above 30 degrees in Canberra. Photo: Melissa Adams

"Holidaymakers are urged to be mindful of the weather conditions in making their travel plans," the bureau's assistant director for weather services, Alasdair Hainsworth, said.

"Fire dangers are expected to be very high to extreme across several states, and the public is urged to check these in their local area."

Much of NSW has high or very high fire danger ratings for Wednesday, while strong wind warnings are also current for Batemans Coastal Waters and Eden Coastal Waters.

Out for temperatures January-March

Outlook for temperatures, January to March 2014.

Weatherzone meteorologist Max Gonzalez said Canberra's forecast would be hot until at least Christmas Day.

"We’re expecting temperatures to soar into the low 30s over the next few days with the peak of the heat on Friday, reaching 37 degrees in Canberra," he said.

Wednesday's forecast top is 32 degrees, before 34 degrees on Thursday.

Outlook for rain over January-March 2014.

Outlook for rain, January to March 2014.

"There will be mostly sunny conditions throughout this period and we will see a trough moving through late Friday and early Saturday, bringing some showers which will be mostly isolated and also a cool relief,"  he said.

"The relief won’t be that great, with temperatures only dropping to the low 30s once again. It will linger in the low 30s through the weekend and then Christmas day is looking to be in the high 20s."

Saturday is expected to reach 32 degrees in Canberra, ahead of 34 degrees on Sunday.

Weatherzone: ACT storm tracker

The heat alert comes as the bureau released its three-month outlooks for temperatures and rainfall for Australia.

They show the odds favoured warmer and drier than normal conditions for the first three months of 2014 for most of eastern Australia.

The forecast suggested the unusual warmth most of Australia had experienced in 2013 would extend well into the new year, and the country was all but certain to post its hottest year in more than a century of records.

Perth's run of five days above 36 degrees from Thursday to Monday was the earliest in the summer for the city in at least 71 years.